1. Technical Field
The present invention relates to a color image processing art for a color output device, particularly to a method and an apparatus for more-accurately adjusting a white point.
2. Prior Art
In recent years, a liquid-crystal display (LCD) have been widely used as a display device for displaying images of a personal computer or television or for various monitors in addition to a CRT. In case of a color display system using the CRT or LCD, it is considered to be ideal to approach colors which can be displayed by the system as, close to natural colors as possible (this is referred to as display color fidelity). Moreover, it is requested for an apparatus using a CRT or LCD to automatically adjust a color or an operator (user) to manually adjust the color in accordance with the state of the apparatus, that is, the environment of the system such as illumination so that an optimum color can be displayed in accordance with each environment (this is referred to as color calibration). Furthermore, it is eagerly requested that the same color can be outputted independently of the type of output device (this is referred to as device transfer characteristic or color matching). These arts are generally referred to as color management. For a color display system that is a high-performance model from the next generation downward, various researches and developments are performed as indispensable technical matters. Particularly, the white point adjustment for adjusting an achromatic color tone in display is very important, which has been realized so far for a color monitor and the like.
In this case, an xy chromaticity diagram shown in FIG. 15 is used to quantitatively handle all natural colors. The diagram expresses the color tone and saturation of a color in accordance with the position of chromaticity coordinates, showing chromaticity coordinates expressed by a horizontal axis x=X.(X+Y+Z) and a vertical axis y=Y.(X+Y+Z), where X, Y, and Z are the three stimulus values of the XYZ color system. All colors sensed by eyes of a person are shown on and inside of the horseshoe-shaped closed curve c. Points R, G, and B in FIG. 15 are points expressing only primary display colors of R (red), G (green), and B (blue) in a specific color display system and thereby, it is possible to express all colors on sides and the inside of a triangle RGB by properly mixing R, G, and B. Moreover, maximum-luminance white can be generally obtained as a mixed color W when bringing R, G, and B into the maximum luminance, which is normally located nearby the intersection of median lines of the triangle RGB.
To design a color display system, a better white point is determined by adjusting the value of the maximum luminance at points R, G, and B or changing positions of the points R, G, and B in FIG. 15. For example, in case of a color display system using an LCD module, it is necessary to consider the spectral radiation of a backlight and a light transmittance through a color filter.
On the other hand, even in case of determining an optimum white point correspondingly to a color display system as described above, several problems occur depending on various conditions including an environmental condition because the determined value is a fixed value. Firstly, there is a problem that the color tone of white differs depending on the environmental illumination where a display is set. For example, when designing a white point at a point having a color temperature of 7000K, the point is felt bluish under an incandescent-lamp illumination at approx. 2800K or yellowish under daylight at 6500K. Secondly, there is a problem that preferable white points are changed depending on the content of a display image. For example, a desired white color differs between an application on MS-Windows and a photograph or dynamic image. Particularly, in case of a photograph image, the desired white color is influenced by the situation when the photograph was taken. Thirdly, there is a problem that the individual difference between users is large for the taste of a white color. For example, Japanese people generally tend to like bluish white. These tendencies are also influenced by an individual difference of ocular function such as an ocular disease. Fourthly, there is a problem on production. For example, in case of an LCD module, white-point producing fluctuation occurs up to approx. xc2x10.03 on xy chromaticity coordinates.
Therefore, it is very significant as a method for solving the above problems to configure a color display system so that a user can adjust the white point at the maximum-gray-scale achromatic color (so-called the full level white) by any method.
Moreover, as an intrinsic problem on a TFTLCD monitor, there is a phenomenon of blue shift in intermediate-gray-scale achromatic colors particularly in a low gray scale. This is a phenomenon that, when making a TFTLCD device display an achromatic color (color in which R, G, and B have the same gray scale), the color becomes more bluish as lowering the gray scale value (that is, chromaticity coordinates shift in the blue-color direction). This phenomenon becomes considerably remarkable depending on the type x of LCD panel. Though it is requested to secure a large angle of visibility for recent LCDs, the phenomenon becomes more remarkable as an angle for a user to view a display (angle of visibility) tilts from the head-on position of the display. When the phenomenon occurs, an achromatic color at an intermediate gray scale is deviated from the setting at the maximum gray scale, no matter how much the white point at the maximum gray scale can be adjusted to desirable chromaticity coordinates (color temperature). Therefore, this causes a new problem.
Though not directly related to the present invention, a prior art for white balance compensation is disclosed in Japanese Published Unexamined Patent Application No. 9-147098 and 7-336700. Japanese published Unexamined Patent Application No. 9-147098 discloses an art for applying LUT conversion to input RGB signals in accordance with the reference white value and reference black value designated by an operator. Moreover, Japanese Published Unexamined Patent Application No. 7-336700 discloses an art for A/D-converting input RGB signals into analog signals and uniforming luminances by an inverse gamma compensation circuit and a white balance compensation circuit.
A display system performing full digital processing has recently become a main stream. For example, in case of LCD display systems except some products, an LCD module uses digital data before the data is inputted to source drivers. Also, an LCD monitor having a built-in LCD module had been provided with an analog interface, performed A/D conversion inside, and transmitted a digital video signal to the LCD module. In recent years, however, some LCD monitors have appeared which are provided with a digital video interface using a low-voltage-differential-type digital data transmission system such as LVDS or TMDS {PanelLink (trademark of Silicon Image Inc.), in which a video signal uses digital data until it is inputted to source drivers after it is outputted from the graphics controller of a system unit. Moreover, in the field of notebook-type PCs, only digital data has been used so far.
When considering the above situation, a technique is required in which the processing for the above white point adjustment can be digitally performed by an efficient circuit.
Under the above background, the present applicant previously proposed an art for adjusting the maximum-gray-scale achromatic color (full white) W and the minimum-gray-scale achromatic color (full black) O to target chromaticity coordinates (Wxe2x80x2 and Oxe2x80x2) as a technique for efficiently performing white point adjustment in digital processing (Japanese Patent Application No. 97183/1999). The art is very superior in performing white point adjustment without increasing a circuit in size. However, when using only the above art, each color of the so-called intermediate-gray-scale achromatic colors located between the target chromaticity coordinates Wxe2x80x2 and Oxe2x80x2 are automatically converted to a color expressed by an integer lattice point approximated by a line segment Wxe2x80x2Oxe2x80x2. This is, so to say, approximation by two points of Wxe2x80x2 and Oxe2x80x2 and a case may occur in which each color of the converted intermediate-gray-scale achromatic color is further deviated from target chromaticity coordinates as the color further separates from an approximation point such as a Wxe2x80x2 or Oxe2x80x2. For example, when adjusting full white and full black so that they have a color temperature (e.g. 5400K) at which they seem to be the most achromatic under the environment, a phenomenon may appear that xc2xc gray automatically converted due to the adjustment becomes bluish but xc2xe gray becomes yellowish. This phenomenon remarkably occurs particularly in a display device using a TFTLCD.
To solve these left problems, it is permitted to increase the number of approximation points and adjust a plurality of achromatic colors other than full white and full black to target chromaticity coordinates, that is, apply the so-called multiple-point approximation to the achromatic colors. However, even if the multiple-point approximation is merely executed, it is performed through conversion of digital R, G, and B values. Therefore, it is necessary to use an efficient conversion algorithm for preventing a logic volume under execution from extremely increasing.
Moreover, conversion in which luminance levels of three primary colors (red, green, and blue) and yellow, magenta, and cyan are not influenced (not attenuated) even if full white adjustment or full black adjustment is performed is effective to keep the number of colors which can be displayed and make the most use of multiple gray scale in accordance with n-bit colors. Moreover, it is preferable that the above conversion can be executed even when executing multiple-point approximation.
Therefore, it is an object of the present invention to realize high-accuracy white point adjustment in which achromatic colors between maximum and minimum gray scales are brought into almost the same chromaticity coordinates by approximating the white point adjustment also for intermediate-gray-scale achromatic colors other than full white and full black in a display system for full digital processing.
It is another object of the present invention to embody; high-accuracy white point adjustment by a simple circuit configuration according to an efficient algorithm in a display system for full digital processing.
It is still another object of the present invention to embody high-accuracy white point adjustment according to multiple-point approximation and perform conversion in which luminance levels of three primary colors (red, green, and blue) and yellow, cyan, and magenta are not influenced (not attenuated) even after the above adjustment.
To solve the above technical problems, a color image processing method of the present invention comprises the, steps of converting an intermediate gray scale input color serving as an achromatic color located between maximum- and minimum-gray-scale achromatic colors into a color having a different color tone according to the converted value being set and slowly decreasing a converted value for input colors other than the intermediate gray scale input color and decreasing the converted value to 0 for an input color in which the value of each color signal configuring the input color is equal to the minimum gray scale value or maximum gray scale value.
Moreover, in addition to the above, the present invention comprises the steps of converting an input color serving as a maximum-gray-scale achromatic color into a color having a different color tone in accordance with a set maximum-gray-scale converted value, slowly decreasing a converted value for input colors other than the input color, decreasing the converted value to 0 for an input color in which the value of each color signal configuring the input color is equal to the minimum gray scale value, converting an input color serving as a minimum-gray-scale achromatic color into a-color having a different color tone in accordance with a set minimum-gray-scale converted value, slowly decreasing a converted value for input colors other than the input color, and decreasing the converted value to 0 for an input color in which the value of each color signal configuring the input color is equal to the maximum gray scale value. Thereby, the present invention is superior in that it is possible to use a conversion system (conversion A) in which three primary colors (red, green, and blue) and yellow, cyan, and magenta are uniformly converted even if white point adjustment is executed for the maximum-gray-scale achromatic color and minimum-gray-scale achromatic color in addition to white point adjustment for an achromatic color whose gray scale value is located intermediately between the maximum- and minimum-gray-scale achromatic colors, and it is possible to express a color closer to reality even for a case of displaying an image using the Dither method.
In this case, a set converted value denotes a parameters for adjustment set by a user. In case of three-point approximation, set converted values include not only a maximum-gray-scale achromatic color point (GL[2nxe2x88x921]) and: a minimum-gray-scale achromatic color point (GL[0]) but, also a fine-adjusted value at a point (GL[2nxe2x88x921xe2x88x921]) located between them. Moreover, in case of five-point approximation, set converted values include fine-adjusted values at GL[2nxe2x88x922xe2x88x921] and GL[2nxe2x88x921+2nxe2x88x922xe2x88x921] in addition to adjustment points for three-point approximation. That is, it is preferable to compensate a white point at the achromatic color of an intermediate gray scale point in powers of 2 because adjustment can be executed in real time while a circuit scale is decreased. The adjustment method by a user can be applied to any purpose such as pop-up of an adjusted value every RGB signal by an on-screen display (OSD) mechanism, direct input of parameters for adjustment as value, or using of adjusted values applied to an image selected by a user in accordance with the comparison method as an adjusted value by displaying a plurality of images (e.g. two images) for white point adjustment at the achromatic color of a specific gray scale point.
Moreover, it is possible to apply an input color to an input color signal generated by a display system such as XYZ display system, or YMCK display system, as well as RGB display system.
Furthermore, a color image processing method of the present invention comprises the steps of converting an intermediate gray scale input color serving as an achromatic color located between maximum- and minimum-gray-scale achromatic colors into a color having a different color tone and slowly decreasing a converted value for input colors other than the intermediate grays scale input color and
decreasing the converted value to 0 for an input color in which the value of any one of color signals configuring the input color is equal to the minimum gray scale value or maximum gray scale value.
According to the above invention, it is possible to realize a conversion system (conversion B) in which luminance levels of three primary colors (red, green, and blue) and yellow, cyan, and magenta are not influenced (not attenuated) after applying white point adjustment to an achromatic color whose gray scale value is located intermediately between the maximum- and minimum-gray-scale achromatic colors.
In this case, when this invention is characterized by converting an input color serving as a maximum-gray-scale achromatic color into a color having a different color tone in accordance with a set maximum-gray-scale converted value, slowly decreasing a converted value for input colors other than the above input color, and decreasing the converted value to 0 for an input color in which the value of any one of color signals configuring the input color is equal to the minimum gray scale value, or by converting an input color serving as a minimum-gray-scale achromatic color into a color having a different color tone in accordance with a set minimum-gray-scale converted value, slowly decreasing a converted value for input colors other than the above input color, and decreasing the converted value to 0 for an input color in which the value of any one of color signals configuring the input color is equal to the maximum gray scale value, it is possible to also apply white point adjustment to the achromatic color of the minimum- or maximum-gray-scale value in addition to the adjustment of an achromatic-color whose gray scale value is located intermediately and realize white point adjustment according to multiple-point approximation by the conversion method using the above conversion B.
It is needless to say that the conversion method using the conversion B can be applied to the content of a set converted value similarly to the case of the conversion method using the above conversion A.
Moreover, a color image processing apparatus of the present invention for applying white point adjustment to input colors in which each color signal is configured bye n bits (n is a natural number equal to or larger than 2) comprises a converted-value input logic for inputting a converted value for an achromatic input color whose gray, scale level is equal to 2nxe2x88x92k or 2nxe2x88x92kxe2x88x921 (k is an integer meeting a range of 1£k less than n) and a converter converting the achromatic input colors whose gray scale levels are equal to 2nxe2x88x92k and 2nxe2x88x92kxe2x88x921 into colors having different color tones in accordance with a converted value input by the converted-value input logic, slowly decreasing a converted value for input colors other than the input colors having the achromatic color gray scale levels of 2nxe2x88x92k and 2nxe2x88x92kxe2x88x921, and decreasing a converted value to 0 for an input color in which the value of each color signal configuring the input color is equal to the minimum gray scale value or an input color in which the gray scale level of its color signal is equal to or more than 2nxe2x88x92k+1xe2x88x921 According to the above invention, it is possible to realize white point adjustment for an intermediate-gray-scale achromatic color located nearby the minimum-gray-scale achromatic color, use the conversion method using the conversion A, and properly compensate a blue shift in a low-gray-scale part even if an image is displayed using the Dither method.
Furthermore, a color image processing apparatus of the present invention for applying white point adjustment to input colors in which each color signal is configured by n bits (n is a natural number equal to or larger than 2) comprises a converted-value input logic for inputting a converted value for an achromatic input color whose gray scale level is equal to 2nxe2x88x92k or 2nxe2x88x92kxe2x88x921 (k is an integer meeting a range of 1£k less than n) and a converter for converting the achromatic input colors whose gray scale levels are equal to 2nxe2x88x92k and 2nxe2x88x92kxe2x88x921 into colors having different color tones in accordance with a converted value input by the converted-value input logic, slowly decreasing a converted value for input colors other than the input colors having the achromatic color gray scale levels of 2nxe2x88x92k and 2nxe2x88x92kxe2x88x921, and decreasing a converted value to 0 for an input color in which the value of any one of color signals configuring the input color is equal to the minimum gray scale value or an input color in which the gray scale level of its color signal is equal to or more than 2nxe2x88x92k+1xe2x88x921. According to the above invention, it is possible to realize white point adjustment for an intermediate-gray-scale achromatic color located nearby the minimum-gray-scale achromatic color, use the conversion method using the conversion B, and improve a blue shift in a low-gray-scale achromatic color part while luminance levels of three primary colors (red, green, and blue) and yellow, cyan, and magenta are not attenuated even after white point adjustment.
Converted-value input logics of these inventions include not only a configuration allowing a converted value to be directly inputted so that a user can input a numerical value or determine an adjusted value in accordance with, the selection of an adjusted image according to the comparison method, but also a configuration allowing an adjusted-value (converted-value) formed by other systems to be inputted through the Internet or the like. Moreover, a slowly-decreasing converted value can be understood as a converted value whose absolute value slowly decreases independently of whether it is positive or negative.
Furthermore, from other viewpoint, it can be also said that the present invention is a color image processing apparatus for applying color conversion to a digital video interface for inputting a digital video signal outputted from a host system and a digital video signal input through the digital video interface without using a look-up table, comprising an adjusted-value input logic for inputting adjusted values at a predetermined point to achromatic colors ranging from the maximum-gray-scale achromatic color to the minimum-gray-scale achromatic color and an output logic for computing a digital video signal input by the digital video interface so as to converge chromaticity coordinates in achromatic colors in accordance with adjusted values input by the adjusted-value input logic and outputting a computed digital value in a pipeline manner.
In this case, the adjusted-value input logic is preferable in that it is possible to apply white point adjustment to maximum- and/or minimum-gray-scale achromatic color(s) in accordance with digital data because the unit is characterized by inputting an adjusted value at the point of the maximum- and/or minimum-gray-scale achromatic color(s). Moreover, the unit is superior in that it is possible to execute white. point adjustment by a simple circuit in real time by outputting a computed digital value in a pipeline manner.
Furthermore, the present invention realizes adjustment by an efficient algorithm according to multiple-point approximation without using a look-up table (LUT). Therefore, it is unnecessary to use an LUT for each target to be adjusted differently from a conventional case and it is possible to realize white point adjustment by a simple circuit configuration.
Furthermore, the adjusted-value input logic is preferable in that it is possible to embody high-accuracy white point adjustment according to multiple-point approximation by a simple circuit configuration because the unit is characterized by inputting an adjusted value at the achromatic-color point at a gray scale level specified in powers of 2 between maximum- and minimum-gray-scale achromatic colors. Moreover, by executing the above multiple-point approximation, it is possible to make white point adjustment even slightly approach ideal white point adjustment in which chromaticity coordinates do not shift over achromatic colors ranging from the maximum gray scale to the minimum gray scale.
Furthermore, the present invention is a liquid-crystal display for displaying an image in accordance with a digital video interface for inputting a digital video signal outputted from a host system and a digital video signal input by the digital video interface, comprising an adjusted-value input logic for inputting an adjusted value at a white point for a digital video signals inputted by the digital video interface, a controller for computing a digital video signal inputted by the digital video interface so as to converge chromaticity coordinates in achromatic colors without using a look-up table and outputting a computed digital value in a pipeline manner in accordance with the adjusted value inputted by the adjusted-value input logic, and a liquid-crystal display section for displaying an image in accordance with a digital value outputted from the controller.
Furthermore, the adjusted-value input logic inputs adjusted values for a white point at a plurality of achromatic color gray-scale levels and outputs results computed so as to adjust a blue shift in a low-gray-scale achromatic color in accordance with the adjusted values inputted by the adjusted-value input logic. More specifically, the adjusted values for a white point at a plurality of achromatic color gray scale levels inputted by the adjusted-value input logic are inputted at intermediate-gray-scale achromatic color gray scale levels between maximum- and minimum-gray-scale achromatic colors. Thereby, it is possible to almost keep the set temperature value by preferred color temperature adjustment at the maximum-gray-scale white point, also in an intermediate gray scale and correct a trouble in white point adjustment intrinsic to a liquid-crystal display.
Moreover, in case of the invention of a color-image processing method, it is possible to add a method for changing conversion methods of the above conversion A and conversion B. Furthermore, in case of the inventions of a color-image processing apparatus and a liquid-crystal display, it is possible to further include a changer for changing the conversion methods of the above conversions A and B. By adding these components, it is possible to use a proper conversion method in accordance with the type of image or the structure of an image-processing system and obtain an optimum output in accordance with user""s taste or status of image.